Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory.
It acts to replace and recover from the activities of living. Its action is (almost always) the opposite of the sympathetic division. It activates the gut for digestion, slows the heart rate, decreases the blood pressure, etc. The autonomic nervous system controls the insides of the body: the viscera or gut.
The autonomic regulation of skeletal muscle blood flow is dominated by the activity of the sympathetic nervous system. A rich adventitial plexus of sympathetic nerve fibers surrounds feed arteries and extends throughout the arteriolar network, including its terminal branches (26, 31, 50).ctivation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure.